(1) Field of the Invention
The present invention relates to a novel, optically active cyclopropane compound and a liquid crystal composition comprising this optically active cyclopropane compound.
This optically active cyclopropane compound is valuable as an optical switch element material, especially a material of a ferroelectric liquid crystal composition.
In the instant specification, by the term "liquid crystalline substance or material" is meant not only a substance showing a liquid crystalline phase but also a substance or material which is valuable as a constituent of a liquid crystal composition though it is not detected that the substance or material shows a liquid crystalline phase.
(2) Description of the Related Art
As the display system using a liquid crystal display element, which is widely utilized in practice at present, there can be mentioned the twisted nematic type (TN type) and the dynamic scattering type (DS type). In these display systems, display is performed by a nematic liquid crystalline cell comprising a nematic liquid cell as the main component. One defect of the conventional nematic liquid cell is a low response speed, and only a response speed of several milliseconds is obtained. This defect is one cause of the limitation of the application range of the nematic liquid cell. Recently, however, it has been found that a high response speed can be obtained if a smectic liquid crystalline cell is used.
It has been clarified that some optically active smectic liquid crystals have a ferroelectric property, and there are great expectations on the utilization of such liquid crystals. Liquid crystals having a ferroelectric property, that is, ferroelectric liquid crystals, are compounds synthesized by R. B. Meyer et al in 1975, which are represented by 2-methylbutyl 4-(4-n-decyloxybenzilydeneamino)cinnamate (hereinafter referred to as "DOBAMBC"). The compounds are characterized as exhibiting a ferroelectric property in the chiral smectic C phase (hereinafter referred to as "SmC* phase") [J. Physique, 36, L-69 (1975)].
N. A. Clark et al found that a high-response speed of an order of microseconds is obtained in a film cell of DOBAMBC [Appl. Phys. Lett., 36, 89 (1980)], and with this finding as a momentum, the ferroelectric crystal has attracted attention as a material applicable not only to a display system such as a liquid crystal television but also to an optical printer head, an optical Fourier converting element, a light valve, and other optoelectronic elements because of high-speed response characteristics.
Since DOBAMBC has a small spontaneous polarization and is a Schiff base, it has poor physical and chemical stabilities. Accordingly, various physically and chemically stable compounds have been investigated as ferroelectric liquid crystalline materials. At present, research work on the development of ferroelectric liquid crystalline materials is concentrated on an enhancement of the high-speed response characteristic, orientation effect, contrast characteristic, memory characteristic, and threshold value characteristic, and optimization of practical properties such as the temperature dependencies of these characteristics.
However, none of the known ferroelectric liquid crystals, when used alone, shows a large spontaneous polarization, a low viscosity, a long helical pitch and an appropriate molecular tilt angle within a broad temperature range including room temperature such that the above-mentioned practically desired properties are manifested. Therefore, practically, attempts have been made to optimize the foregoing characteristics by mixing several compounds such as a compound having or inducing a large spontaneous polarization, a compound having a low viscosity and compounds having reverse helical pitches. The incorporation of a ferroelectric liquid crystal showing a ferroelectric characteristic within a broad temperature range or a smectic C liquid crystal which is not chiral is effective for obtaining a liquid crystal composition showing a ferroelectric characteristic within a broad temperature range. In connection with liquid crystalline materials used for ferroelectric liquid crystal compositions, it is considered necessary to optimize the physical properties by selecting (1) a compound having or inducing a large spontaneous polarization, (2) a compound considered from the skeleton to have a low viscosity or a compound not degrading the liquid crystalline property of a compound considered from the skeleton to have a low viscosity when both are mixed, (3) a compound having a short helical pitch and capable of unwinding the helical pitch by the addition thereof in a minor amount, and (4) a liquid crystalline substance showing a ferroelectric property within a broad temperature range, among a great: number of compounds differing in the skeleton and optically active group, and mixing them together.
The presence of an optically active group is indispensable for a compound to be a ferroelectric liquid crystal. Amyl alcohol, octyl alcohol and the like are mentioned as the known optical activity source. Although these alcohols are easily available, they are monohydric alcohols having poor reactivity, and chemical modification in the vicinity of the optically active site is limited and the molecular design of a liquid crystalline substance is therefore difficult. As a typical instance of this chemical modification, there can be mentioned only the carbon number-increasing reaction proposed by A. Hallsby et al [Mol. Cryst. Liq. Cryst., 1982 (62), L61] or J. W. Goodby et al [Mol. Cryst. Liq. Cryst., 1984 (110), 175-203].
As the effect of the optically active group or the neighbouring substituent, there can be mentioned the steric factor and electronic factor of the asymmetric source. As the effect on the physical properties of the liquid crystalline material, the former factor has an influence on the symmetry of the liquid crystalline molecule and, for example, control of the range of temperatures showing the liquid crystalline phase becomes possible. The latter factor gives a change to the optically active group or the neighbouring dipole moment and control of the direction and magnitude of the spontaneous polarization, the length and direction of the pitch and the viscosity becomes possible Accordingly, if the effects of the optically active group or the neighbouring substituent are effectively controlled, the molecular design of a compound satisfying the requirements of various characteristics such as temperature range, response speed, viscosity and pitch length, which are imposed on a ferroelectric liquid crystalline material ensuring realization of large-scale liquid crystal panel display, or the design of a composition of such compounds will be easily accomplished.